Lapping is a well-known process of abrasion metal-removal or machining for smoothing or polishing surfaces to a high degree of refinement or accuracy using a loose abrasive lapping compound. The lapping compound is often in a liquid suspension or semi-liquid form, and is called lapping slurry.
The present invention arose out of an effort to improve the cleanliness of parts that had been lapped to certain thickness and roughness specifications, and subsequently ultrasonically cleaned. In at least one instance ceramic wafers of alumina substrates that had been ultrasonically cleaned subsequently had metallized microcircuits applied for use as automobile sensors. Because of suspected surface contamination from the lapping operation, the microcircuits were not adhering properly to the contaminated ceramic substrates. Because it was suspected that residual lapping swarf or lapping carrier residue was interfering with the adherence of the microcircuit, efforts were initiated to more completely clean the workpieces after lapping. However, additional cleaning efforts were not entirely successful in eliminating the problem, and it was then determined that some contaminant adhesive was causing the lapping swarf to adhere to the workpieces. It was later determined that the lapping slurry in use at the time was a major contributor to the contamination problem. The slurry in use consisted of water, triethanolamine, 15-25 um diamond, and xanthan gum.
Xanthan gum is a commonly-used thixotropic component that suspends the diamond abrasive until it arrives on the lapping table. Shear forces then thin the slurry allowing the diamond to drop out onto the ceramic parts. It was found that under the shear forces, the xanthan gum formed into balls which were subsequently smeared onto the ceramic surfaces during the lapping process. This smeared gum acted as an adhesive for the particulate removed from the ceramic surfaces, making cleaning very difficult.
The lapping slurry in use also contributed to other operating problems regarding lapping machine cleanup. When machines are cleaned at the end of a work shift, the large cast iron tables are left unprotected overnight. In the morning, surface rusting is quite extensive, requiring cleanup prior to lapping. The presence of rust on the surface of the ceramic wafers is also a reason for rejection. A second problem also involves the xanthan gum lapping slurry. When lapping machines are unused during weekends, the xanthan gum-alumina swarf sets up like a mortar on the lapping table, and this often causes the first set of lapped parts to jam the machine on Monday morning.
In an effort to eliminate the problems described above, various slurry replacements were tried. One of the replacements was a thixotropic corn starch mixture. While this slurry gave acceptable lapping rates, it left intractable deposits on the lapping machine and on the ceramic workpieces.
Another slurry tried was one made of glycerine, triethanolamine, diamond abrasive, and water. This slurry gave excellent lapping times and cleaned up superbly. In addition, this slurry facilitated clean-up of the lapping table. It did not make "mortar" on the lapping table during off shifts. Moreover, residual triethanolamine inhibited surface rusting of the lapping tables. The only problem encountered with this slurry was that it did not suspend the diamond adequately. It was not thixotropic and had to be blended at a low viscosity suitable for lapping. Constant vigorous stirring was required to keep the diamond suspended. Silica gel and other viscosity enhancers proved to be unacceptable.